Microwave frequency stabilizer



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Aug. 7, 1951 MICROWAVE FREQUENCY STABILIZEE Patented ug. 7,

y l :MiCnowAyarREQUENoY s'rAILIzER Louis iivsmuuin; Nutley, NJ., 'ana James Brown HornerPKuper; Bayport, N. Y., assigvnorsto Federal.'l Telephone 4and Radio Corporation, New

York; .-N; Y.;,a lcor-poratiori of Delaware 'Y A1-mueaupa April s, 1948seria1N0. 19,688l

' This' invention relates to microwave't'or' ultrahigh-frequency oscillators, and moreparticula'rly it relates to the frequency stabilization of such oscillators.

A principal.A object ofthe invention is to improve the performanceofimicrowave oscillators having a-frequencyestabilizingarrarigement of 'fthe so-called Magic T type.

A featureA of. the invention relatesfto a microwave oscillator stabilizing system employing Aa frequency-sensitive arrangement having `a pair of crystal detectors vwhich are respectivelyY coupled to the oscillatoriiby.` respective Magic T wave guides, and means are provided in the said T'sections to avoidtheundesirable effects of` reflectionfrom thecrystalsther'ein.

Another featurev relatest'o' a" ymicrow'a'v'e oscillator stabilizing system employing a solcalled Magic T frequency discriminator.,.togetherwith special attenuation matching p'a'ds for-they crystals in each "T" section to prevent' e1`fra`.ticlocl:` ing-in of the oscillator at an undesired frequency, and to insure the maintenance of symmetrical frequency discriminator characteristic curves.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood, by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a composite structural and schematic wiring diagram, partly isometric, of a microwave oscillator system according to the invention.

Fig. 2 is an equivalent schematic circuit diagram of the system of Fig. 1.

The microwave generator according to the present invention comprises any well-known micro- Wave oscillator whose frequency can be controlled by the magnitude of a D. C. control signal, which in turn is determined by the coaction of a double Magic T frequency-sensitive arrangement of the wave guide type. A complete description of the operation of the Magic T coupler may be obtained in Radiation Laboratory Report 662 entitled Microwave Frequency Discriminator.

When such a Magic T is used as a frequency discriminator, it is used as a bridge to compare the reections from a short-circuited stub wave guide, with that of a resonator such as an adjustable cavity resonator located at the end of another line which is an odd number of eight Wave lengths long. An exposition of the general principles f the frequency discriminator will comms. (cinto-ese) 2 also begfround' in Radiation' Laboratory Report 662' entitled Microwave Frequency Discrimi'r'iaetori Thus as 'shown in Fig. 1, the waves from' the microwave oscillator 5 are'cou'pled into' a Magic T comprising input arm' `(i, lateralar'ms I vand 8, and opposite arm 9, so that one-halfithe wave energy travels towards theA right andenters another' Magic T comprised of input armA I D, lateral arms I I and I2, and opposite arm L3. 'The' arm II is short-circuited at the' end I4, while thearm' I2 is terminated in the tunable'highmQ cavity resonator' i5. Crystal detectors` IS," IIi are located respectively in armsV 9" andr` I3j and; they arev preferably ofthe silicon' point-contact rectiertype.

It will be' assumed that the'oscillat'or 5"is re' quired to generate at a stabilized frequency F, and'therefore the cavity' I5`is tuned byknob I8vr tolresonate at this frequency. The arm I-I, isof a lengthl, andthe arm I2is ZLFA/Swhere A is the wave length ofi theI` waves. citation frequencies far above or below the said resonant frequency, the cavity I5 appears as a short circuit, and the reflected waves are shifted by vr/Z in phase relative to one another as they reconverge on the T junction ID-I I-I2.

On the other hand, in the vicinity of resonance the cavity I5 gives rise to a reflection in the same or opposite phase to the reflection from end I4, so that at resonance, waves again are impressed on the T junction IU--II-IL The operation of the discriminator will be apparent from a study of Radiation Laboratory Report 662, loc. cit.

In order to achieve the desired symmetry of operation, it is necessary in tuning-up, to adjust the outputs of the detectors I6, I1. However, we have found that with the double-T arrangement shown in Figs. 1 and 2, difficulties are often encountered, amongst which may be mentioned erratic locking-in of oscillator V5 at some undesired frequency. While it is always possible to obtain some stabilization of oscillator 5, as long as there is a region in which the voltage output of amplifier I9 changes rapidly in the required direction with change of frequency, unless the discriminator curve is symmetrical and unless there is a definite ratio between the currents from the crystals I6 and I7, it is difficult to prescribe a tuning-up procedure.

We have found both by experiment and mathematical analysis, that reflections from the crystals I6, I'I are of great importance in giving rise to such abnormal discriminator curves, and most particularly in upsetting the expected two- Consequently forexf to-one ratio between the powers incident on the crystals when the cavity I is detuned. Unless this two-to-oneV ratio is known, it becomes difficult to make the adjustments to insure that oscillator 5 is correctly stabilized and that it is operating at the center of the range over which stabilization can be expected. To minimize, and in certain cases to eliminate, these difficulties,- according to the invention there are interposed between the crystals I6, I1 and their respective T junctions, suitable wave attenuator or resistor pads 22, 23. These pads can be of any well-known design and may take the form of tapered cards such as shown in Figs. l and 2 vand are located respectively within the arms 9 and I3 in front of detectors I6, I1, and should be of such a resistance at the operating frequencies that the loss is great enough to reduce the reflection from any crystal selected at random to a negligible value, for example to a standing wave voltage ratio of approximately 1.2 and not substantially above 1.8. The loss required in the arm 9 (which normally receives only one-half the power received by crystal I1) may, for eX- ample, be Ymade three decibels less than that in arm I3, so that the oil-resonance currents are approximately equal.

While we have described above the principles of our invention in connection with speciiic apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention.

What is claimed is:

l. A microwave oscillator frequency stabilization arrangement, comprising a first Magic T junction, a second Magic T junction, each junction having an input arm, a pair of lateral arms and an arm opposite to the input arm, a crystal detector in each of said opposite arms, a direct current amplifier for amplifying the difference between the crystal outputs, an oscillator aseaese f* 4 to be stabilized and coupled to the input arm of the rst Magic T, one of the lateral arms of the rst Magic T being coupled to a load circuit, and the other lateral arm of the rst Magic T being coupled to the input arm of the second Magic T, one lateral arm of the second Magic T being terminated in a tunable resonant cavity and the other lateral arm of the second Magic T being terminated in a short circuit, and means for reducing the wave reflections from each of said detectors comprising an -attenuator pad adjacent to and in front of each detector arranged for reducing the wave reilection therefrom.

2. A microwave oscillator frequency stabilization arrangement according to claim 1 in which the said attenuator pads reduce the reection from each crystal to a standing wave voltage ratio of between 1.2 and 1.8.

3. A microwave frequency stabilization arrangement according to claim l in which said cavity is tuned to the desired stabilized frequency of the oscillator.

LOUIS D. SMULLIN. JAMES BROWN HOMER KUPER.

REFERECES CITED The following references ,are of record in the file of this patent:

UNITED STATES PATENTS Name Y Date Sontheimer et al. July 8, 1947 OTHER REFERENCES Number 

